The optical disc has become increasingly popular in recent years. Audio compact discs, video discs, and digital optical recording for computer data storage are in widespread use. Optical discs feature fast random access, high information density, and long wear because there is no mechanical contact in read/write processes.
Generally, optical information storage on discs works in the following way. A laser beam, modulated by an information signal, writes on the disc by thermally altering the disc medium in some way to produce an optical contrast (typically from a change in reflectivity of the medium). A read-out laser beam follows the information-laden alterations in the disc and a photodetector converts the changes in optical contrast to electrical signals. Audio and computer data signals are typically digitally stored while video signals are stored in frequency-modulated analog form.
Optical contrast may be achieved by changes in reflectivity, signal phase, or state of polarization. Reflectivity changes result from the scattering of light off of an altered physical feature of the disc such as a hole or a bump, or by exposing a reflecting layer through a hole in a non-reflecting surface. Signal phase changes result from the changed path length of the light beam due to the altered relief of the disc. The signal phase relationships of course also affect the reflectivity because of changes in intensity of the reflected beam due to constructive or destructive interference. Information-bearing polarization changes result from a rotation of the plane of polarization due to an altered magnetization of the disc (Faraday or Kerr Effects).
Optical discs basically consist of a thin photosensitive film deposited on a supporting substrate. There also may be intermediate layers such as dielectric spacers for increasing the path length to produce constructive interference, polymer layers for producing gas for bubble formation, mirrored surfaces for increasing reflectivity, and triggering layers to implement or enhance reactions.
Recording information thermally on optical discs traditionally has been divided into five categories--materials depletion, materials deformation, materials interaction, phase transitions, and thermomagnetic (or magneto-optic)--although the physical phenomena occurring in these categories often overlap. The materials depletion category (often referred to as ablation) includes hole-burning and all ablative processes. The materials deformation category includes bubble formation and textured structure recording methods. The materials interaction category includes alloying of bilayers and segregation of compounds. Phase transitions include crystalline-to-amorphous and vice-versa transitions. The thermomagnetic category involves ferro- and ferrimagnetic materials subjected to external fields and irradiated by plane-polarized light beams. The essential goal of each method is to produce detectable optical contrast in the disc. Each of these recording methods will be described below.
In materials depletion optical recording, geometrical changes are induced by evaporating (ablating) a thin layer to produce optically detectable effects. The hole-burning method typically utilizes a low melting temperature metal or semi-metal thin film over a quarter wavelength spacer layer and mirrored substrate. The laser beam simply melts the thin layer material and a hole forms because of the difference in surface tension between the thin film and the underlying spacer layer or substrate. Because of the difference in reflectivity between the thin film and the mirror exposed by the hole, an optical contast is produced which reflects the information in the signal-modulated laser beam. For an example, see U.S. Pat. No. 4,097,895 to Spong.
In materials deformation optical recording, geometrical changes are induced by melting a thin layer to produce optically detectable effects. The bubble formation method uses a polymer-metal bilayer. The laser beam decomposes the polymer and the resultant emitted gas forms a bubble beneath the metal which has a diameter approximately equal to the size of the laser focused spot. The proper choice of material and layer thickness prevents the bubble from bursting. Light scattered from the bubble or transmitted through the bubble with signal phase changes will produce the desired optical contrast. See, for example, U.S. Pat. No. 4,300,227 to Bell.
The textured structure method utilizes a roughly spiked surface with local dimensions that are much smaller than the wavelength of the laser beam. The laser heats the rough surface causing the spikes to melt and the surface to flatten. The flat surface reflectivity is much higher than that of the textured surface thereby producing a strong optical contrast. See Suh, et al., "Morphology Dependent Contrast Measurements of Microscopically Textured Germanium Films", SPIE Proceedings, 382, 199 (1983).
In materials interaction recording, the optical constants of the sensitive layer are changed resulting in changes in reflectivity. Bilayer structures are known which alloy as a result of laser heating. The alloy has a different reflectivity from the separated constituents. This method is very sensitive since the laser beam need only trigger the alloying exothermic chemical reaction. See Ahn, et al., "High Sensitivity Silicide Films for Optical Recording", CLEO Proceedings, 140 (1982).
The segregation method is just the opposite of alloying in that compounds are separated by the heating action of the laser beam to cause, for instance, crystallization of one constitutent after segregation resulting in a high reflectivity. See Akahira, et al., "Sub-Oxide Thin Films for an Optical Recording Disk", SPIE Proceedings, 329, 195 (1982).
In phase transition recording, the crystalline-amorphous and vice-versa transitions produce changes in the optical density of the strucuture, resulting in changes in the reflectivity. See, for example, U.S. Pat. No. 4,091,171 to Ohta, et al.
Thermomagnetic recording utilizes the intrinsic magnetic field of ferro- and ferrimagnetic materials. To write on the disc, a small external magnetic field is applied with a direction opposite that of the intrinsic field of the ferro- or ferrimagnetic materials used in a layer on the disc. Heating by the laser beam above the Curie temperature results in the magnetic moments of the layer lining up with the external field direction. A linearly-polarized read-out laser beam will have its plane of polarization rotated as a result of reflection by the disc surface at the locations where the magnetic field has changed direction (Faraday or Kerr Effects). A polarizer with its plane normal to one of the states of polarization can decode the signal. See Bouwhuis, G., Principles of Optical Disc Systems, 222, Adam Hilger Ltd. (1985).
Typically, optical discs have grooves already cut into the disc. The space between the grooves may be called the "land." Information may be stored in "pits" (the holes from hole-burning) and/or "bumps" (the bubbles from bubble formation) on the land or in the grooves.
In all applications of information storage devices, the greater the information capacity, the greater the capability of the machine using the device and the smaller the whole system may be. The importance of miniaturization has been demonstrated in the development of audio, video, and computer data storage devices. Magnetic tape became a major consumer product only after it was miniaturized into cassettes. Early video tape machines were bulky and could store only an hour of programming. Early computers used punched cards, paper tape, and reels of magnetic tape which presented severe physical storage problems. The application of data compression methods made possible the now common floppy discs, hard discs, and optical discs for computers, and automobile cassette stereos, compact disc players, and mini-VCRs.
One way of increasing the data density of a data record is to change the way that information is encoded. For example, if the well-known binary coding system used in computers (where "0" and "1" are the bits) were converted to a ternary coding system (using "0", "1", and "2" as bits), the amount of data capable of storage would increase. For instance, in magnetic recording devices such as computer disc drives, the bit density may be increased by using a ternary alphabet instead of the common binary alphabet. Data is typically encoded in a stream of bits on a record track. A quantity called the density ratio is a measure of the amount of information capable of being stored in a given length of track. The density ratio equals the code rate times the minimum distance between two consecutive non-zero symbols. The code rate is just the number of data bits divided by the number of code symbols. Using a ternary alphabet instead of a binary alphabet allows an increase in the code rate while maintaining the minimum distance. Thus, the density ratio is increased merely by converting to a ternary system. For details, see Jacoby, G., "Ternary 3PM Magentic Recording Code and System", IEEE Transactions on Magnetics, Vol. Mag-17, No. 6, 3326 (1981).
In the binary code, the minimum number of bits, "a", in each word for storage of information having "M" words is EQU 2.sup.a =M.
When a ternary alphabet is used, the same information can be stored in a word that uses "b" bits; that is, as long as EQU 3.sup.b .gtoreq.M,
the minimum is when EQU 3.sup.b =M,
therefore, EQU 2.sup.a =M=3.sup.b
or EQU a/b=ln(3)/ln(2)=1.58.
In other words, the amount of space in the medium that is required for storing M words using a bianry code is 1.58 times the space required when using a ternary code. This is an increase in capacity of over 50%.
In general, the equation for the factor increase in data density R as a function of number of reflectivity states N is EQU R=ln(N)/ln2.
There have been various attempts to increase the information-carrying capacity of optical discs. There are, for instance, a number of known schemes where recording pits of different lengths may be used to represent specific combinations of data bits.
In another scheme, U.S. Pat. No. 4,090,031 to Russell discloses an optical disc with multiple layers for data storage. Specific layers are accessed by either focusing or filtering a write/read light beam. Transparent spacer layers are used to separate the data layers. The non-accessed layers must also be sufficiently transparent when out of focus to enable unimpeded reading of a selected layer. If white light is used as the write/read beam, filters may be used to access specific data layers. Russell's invention requires a special structure of multiple data storage layers made of transparent materials and a light beam capable of writing on and reading from the desired layer by focusing on that layer alone and not disturbing other layers. In the case of using filters for data layer selection, the use of a white light write/read beam for optical disc recording is not common. White light sources have not been used because only the concentrated power of lasers provide the required signal-to-noise ratios from extremely small areas on the disc and for the extremely short read time intervals.
Another scheme to increase data density uses different wavelengths to record, and filters to read the desired data. A photochemical medium can store many bits of information at each location using certain substances with wavelength-dependent absorption. To write, a laser is tuned to any of up to 1000 discrete frequencies. At each frequency, a small portion of the molecules at the irradiated site undergo chemical tranformations which rob them of their abiity to absorb again at that frequency. Such a recording structure is called a frequency-spatial memory. See Isailovic, J., Videodisc and Optical Memory Systems. 319, Prentice-Hall (1985). This scheme requires special photochemical materials and structures for the optical recording medium, tunable write lasers, scanning read lasers, and a new control system for implemention.
U.S. Pat. No. 3,902,010 to Goshima discloses an optical disc with two data layers of different sensitivity. The high sensitivity layer stores the modulated, information-carrying light beam signals and the low sensitivity layer stores the unmodulated guide beam which controls the light beam. Although there are layers of different photosensitivity, Goshima's invention provides a means for guiding the light beam and storing data on optical discs; it does not increase data capacity.
It is therefore an object of the present invention to provide a method for increasing the information storage capacity of optical discs.
A further object of the present invention is to provide a method for increasing the information storage capacity of presently existing optical disc structures without the need to modify either the discs or the wavelength of the laser beam used to record on, and read out from, the disc.
Another object of the present invention is to provide an optical recording structure with different states of optical contrast that are sufficiently stable for optical information storage uses.
These and other objects of the present invention will become obvious to those skilled in the art from the following detailed description of the invention and the accompanying drawings.